"...Aristophanes tells a tale in Plato's Symposium of how there were originally three sexes: man, woman, and a union of the two. These primeval creatures were round and had twin sets of arms and legs, and could essentially cart-wheel around. They grew strong and attacked the gods, attempting to scale heaven itself.
Fearing that the humans had grown too strong, Zeus decided to split them down the middle, dividing and diffusing humans into the form we have today. Love, Aristophanes continues, is the reuniting of the original couple:
And when one of them meets with his other half, the actual half of himself, whether he be a lover of youth or a lover of another sort, the pair are lost in an amazement of love and friendship and intimacy, and one will not be out of the other's sight, as I may say, even for a moment: these are the people who pass their whole lives together, and yet they could not explain what they desire of one another."
Michael Bruce, Jan 2011, Psychology Today